1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer, a copying machine and the like. More particularly, it relates to an image forming apparatus in which a sheet is discharged onto a discharge portion after the sheet is temporarily held.
2. Related Background Art
In conventional image forming apparatuses, generally, a sheet on which an image is recorded by a recording means is discharged onto a discharge portion immediately after recording. Such an arrangement has widely been used in image forming apparatuses of the ink jet recording system which have recently been popularized.
In the ink jet recording system, the recording is effected by discharging ink droplets. Regarding recording density of normal recording such as a standard document or a table, there is no problem, but, when recording density is great (such as color recording), a time period sufficient to fix the ink is required. In a color image forming apparatus, if an image forming speed is increased, before the ink is fixed to the sheet discharged on the discharge portion, a next sheet is discharged onto a discharge portion. As the result, the imaged surface of the preceding sheet discharged on the discharge portion is rubbed by a tip end or a back surface of the next sheet, thereby distorting the recorded image.
To avoid this problem, a technique for promoting the drying of ink by using heat from an infrared ray heater has been proposed. However, when the infrared ray heater is used, the entire apparatus is made complicated and expensive. Thus, a technique not using the heater has also been proposed.
For example, as shown in FIG. 11, a pair of opposed and spaced side rail members 50a, 50b are supported for rotation around respective pivot points 51a, 51b. Further, vertical walls 52a, 52b are provided with recessed portions 53a, 53b through which the side rail members 50a, 50b are rotated in directions shown by the arrows X.
With the arrangement as mentioned above, a recorded sheet S is conveyed by a convey means (not shown) in a direction perpendicular to the plane of FIG. 11.
At this point, as shown in FIG. 11, the side rail members 50a, 50b are held in a closed position so that they can support the sheet S and hold the sheet above a discharge portion 54a of a discharge tray 54. When the sheet S is discharged, the side rail members 50a, 50b are rotated in the directions X to an open position where the side rail members do not interfere with the sheet S, thereby permitting the dropping of the sheet onto the discharge portion 54a.
In this way, the ink image recorded on the sheet S rested on the discharge portion 54a is dried while the image is being recorded on the succeeding sheet, thereby preventing deterioration of the recorded image.
However, in the above-mentioned technique, since the operation of the side rails must be synchronized with the sheet conveying operation, the mechanism may be made complicated. And, if a drive source such as a motor for achieving the synchronism is added, the entire apparatus will be made more expensive.